In a grinding apparatus of the kind in consideration, also referred to as a disc refiner, for defibration and additional grinding of the material, the pulp raw material when passing from the center to the periphery between the grinding discs is acted upon mechanically by the ridges and grooves of said discs. In order to obtain sufficient defibration and impart to the pulp good properties in other respects, it is often necessary to supply great quantities of mechanical energy per ton of ground produce. In many cases, more than 1,000 kWh or even more are supplied for each ton of produced pulp calculated as bone-dry. When grinding the raw material, such as wood chips, for example, which has a relatively high content of dry substance, a product having longer fiber length and, at the same time, great tear strength, has proved to be obtainable. Usually the grinding is carried out with material having a dry content of 20% and more.
The energy supplied in the grinding operation is converted mainly into heat, which in turn, if the dry content of the grinding produce is high, only to a minor degree can be absorbed by heating the pulp material and the entrained water. The major portion of the generated heat causes evaporation of water with consequent generation of great quantities of steam.
In the defibration and additional grinding of ligno-cellulose material in disc refiners for production of paper pulp, usually one ton of steam and even more is generated per ton of produced pulp calculated as bone-dry. The volume at atmospheric pressure of the generated quantity of steam may amount to 2,000 cubic meters or more per ton of produced pulp. This steam quantity is generated in the disc refiner in the grinding gap or interspace between the grinding discs and must in some way be discharged from the zone where it is gennerated. A great portion, often the major portion, of the steam flows radially outwards towards the outer periphery of the grinding interspace. To a great extent the flow takes place through grooves in the grinding surfaces and often at velocities on the order of some hundred meters per second. Another portion of the steam flows out from the grinding zone in the direction towards the center of the grinding discs.
This high steam velocity results in the grinding produce being carried along with the steam, especially in the outer portion of the grinding zone, thereby causing the grinding produce or grist to be blown out from the grinding zone before it has undergone a satisfactory grinding operation. Consequently, the outermost portion of the grinding zone will work less effectively. The farther out towards the periphery, the greater are the steam quantities generated and the higher are the velocities imparted to this steam.